Blocking Effect of an Immuno-Suppressive Agent, Cynarin, on CD28 of T-Cell Receptor

Purpose Cynarin, a potential immunosuppressant that blocks the interaction between the CD28 of T-cell receptor and CD80 of antigen presenting cells, was found in Echinacea purpurea by a new pharmaceutical screening method: After Flowing Through Immobilized Receptor (AFTIR; Dong et al., J Med Chem, 49: 1845-1854, 2006). This Echinacea component is the first small molecule that is able to specifically block “signal 2” of T-cell activation. Methods In this study, we used the AFTIR method to further confirm that cynarin effectively blocked the binding between CD80 of B-cells and CD28 of T-cells, and provide details of its mechanism of action. Results The experimental results showed that cynarin blocked about 87% of the CD28-dependent “signal 2” pathway of T-cell activation under the condition of one to one ratio of T-cell and B-cell in vitro. Theoretical structure modeling showed that cynarin binds to the “G-pocket” of CD28 (Evans et al., Nat Immunol, 6:271-279, 2005), and thus interrupts the site of interaction between CD28 and CD80. Conclusions These results confirm both that AFTIR is a promising method for screening selective active compounds from herbal medicine and that cynarin has great potential as an immuno-suppressive agent.